1. Field of the Invention
The present invention relates to a hydraulic type brake system which includes a unit for adjusting a back pressure in a turn driving hydraulic circuit mounted in a construction heavy equipment such as an excavator, etc., and in particular to a hydraulic type brake apparatus which is used together with a relief valve installed various actuator driving hydraulic circuits for adjusting a back pressure in a lower flow side of a relief valve for thereby adjusting an opening pressure of a relief valve.
2. Description of the Background Art
A relief valve is directed to draining a part or all of a hydraulic oil in a direction of a hydraulic pressure tank for thereby maintaining a pressure of a hydraulic pressure circuit below a set value when the maximum pressure of a hydraulic circuit reaches at a set value of a valve.
FIG. 1 is an inner cross-sectional view of a direct driven type relief valve which is an example of a relief valve.
The relief valve includes an inlet 11 connected to a hydraulic circuit, a poppet 12 adapted to limit a flow of a hydraulic oil flowing from the inlet, a spring 13 adapted to elastically bias a state that the inlet 11 is closed to an initial state by pressurizing the poppet 12, a pressure adjusting unit 154 adapted to adjusting an elastic force of the spring, and an outlet 15 adapted to returning the hydraulic oil to a hydraulic tank.
In a dynamic relationship that a relief valve is operated, the relief valve is operated based on a combined force between a force applied to a water pressure area of the poppet 12 by a pressured of a hydraulic oil in the side of the inlet 11, namely, a force of the spring 13 and a force applied to a water pressure area of the poppet 12 by a pressure of a hydraulic oil in the side of the outlet 15.
Therefore, in the case that the relief valve is opened, the above state represents that the force by the pressure of the hydraulic oil in the side of the inlet 11 is larger than the force of the counter action, and in the case that the relief valve is closed, the above state represents that the force by the pressure of the hydraulic oil in the side of the inlet 11 is smaller that the force of the counter action.
Therefore, the combined pressure of the pressure by the force of the spring 13 and the pressure of the hydraulic oil in the side of the outlet 15 is an opening pressure. When the pressure of the hydraulic oil in the side of the inlet 11 exceeds the opening pressure, the relief valve is opened.
FIG. 2 is a hydraulic circuit of the conventional hydraulic brake.
As shown therein, there are provided a hydraulic pump 21, a hydraulic motor 27 driven by a hydraulic oil discharged from the hydraulic pump 21, and a direction changing valve 22 which is switched when a signal pressure is applied to a flowing path between the hydraulic pump 21 and the hydraulic motor 27 for thereby controlling a flowing direction of a hydraulic oil supplied to the hydraulic motor 27.
There are provided a pair of relief valves 23 which are installed in a pipe path connecting main pipe paths 25 and 26 in parallel which connect the direction changing valve 22 and the hydraulic motor 27, and a pair of check valves 24 which are installed in another pipe path connecting the main pipe paths 25 and 26 in parallel. An intermediate portion of the pipe path in which the relief valve 23 is installed and an intermediate portion in which the check valve 24 is installed are connected each other and opened.
The operation of the hydraulic circuit will be described. The pressure oil discharged from the hydraulic pump 21 is switched in its supply direction by the operation signals 30a and 30b of the hydraulic motor, and the pressure oil flowing through the direction changing valve 22 is directed to driving the hydraulic motor 27 based on the switched direction. The pressure oil which drives the hydraulic motor 27 flows to the hydraulic tank 28 through the direction changing valve 22.
When the direction changing valve 22 is switched to an intermediate position for stopping the hydraulic motor 27, the hydraulic motor driving circuit connected to the direction changing valve 22 becomes the closed circuit 29, so that a high pressure hydraulic oil adapted to rotate the hydraulic motor 27 stays in one side of the main pipe paths 25 and 26, and since the hydraulic motor 27 has a certain rotational force by the inertia force, the hydraulic motor 27 has a role of the hydraulic pump.
Therefore, the hydraulic pressure of the main pipe path 25 or 26 which has a high pressure hydraulic oil is sharply increased, and when the hydraulic pressure exceeds a set pressure of the relief valve 23, the relief valve 23 is opened, and the hydraulic motor 27 is stopped while the pressure oil circulates in the closed circuit 29.
The relief valve performs a function for limiting the maximum pressure of the hydraulic circuit based on the above operation and operates a based on a counter force with respect to the rotational force by the inertia force in the stopped state for thereby implementing a hydraulic braking effect. Therefore, in order to enhance a braking effect, it is important to increase the set value of the relief valve. In this case, it is not needed to increase the set value of the relief valve for only obtaining a braking effect. Namely, in the relationship between the turning system and the hydraulic circuit, a proper set value of the relief valve is determined.
It is difficult to adjust the set value of the relief valve for implementing a desired braking effect based on the above limit with respect to the set value of the relief valve. In addition, an over move-back phenomenon occurs in the turn due to a lack of a braking force during a stop of turn in a construction heavy equipment which uses the turning system, so that it is difficult to operate for thereby causing a safety accident.